Tuning aid for high fidelity radio receiving systems and the like



May l9, 1953y B. s. vlLKoMl-:RsoN

TUNING AID FOR HIGH-FIDELITY RADIO RECEIVING SYSTEMS AND THE LIKE Flled Aprll 50 1949 INVENTOR BENJAMIN S.\/'1IKEM1:RS

ATTORNEY Patented May 19, A1953 TUNING AID FOR HIGH FIDELITY RADIO RECEIVING SYSTEMS AND THE LIKE Benjamin S. Vilkomerson, Camden, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application April 30, 1949, Serial No. 90,674

(Cl. Z50-20) Claims. 1

'Ihis invention relates to tunable high fidelity radio receiving system and the like, andhas particular reference to improved signal-responsive tuning aids for facilitating the accurate tuning of such systems to selected signals.

Most radio receiving systems, such as used in broadcast and similar receivers at the present time, operate on the superheterodyne principle to select and to convert received signal-modulated radio frequency carrier waves into correspondingly modulated intermediate frequency carrier waves, whereby additional signal amplincation may be effected with optimum eiciency before the intelligence signals are recovered from the carrier waves by a signal detector. In order that the sounds which are linally reproduced from the intelligence signals have the desired fidelity, it is necessary, among other things, to provide intermediate frequency signal amplifying stages having wide frequency .pass bands. In order to produce the desired xed intermediate frequency, it is necessary, in tuning the receiver, to properly adjust the frequency of a local oscillator.` When the oscillator is adjusted to produce a frequency whichv can be combined with the received carrier wave frequency to produce exactly the predetermined intermediate frequency, the receiver will be tuned for optimum performance.

Invorder that the soundl reproduction may be of high fidelity, it is necessary not only to provide ampliiication for a relatively widebandA of signal frequencies, but also the frequency respense of the amplifiers, such as the intermediate i frequency amplifier, must be such that there is no material discrimination between any of the signal frequencies which it is desired to convert into sound eects. Consequently, such amplifiers, particularly for high fidelity modulated signal or broadcast reception, are designed to have what is known as substantially a flat response characteristic over the frequency pass band. Such a response characteristic generally provides a frequency vs. amplitude, or response, curve which is either flat-topped or double-peaked.

Generally, in tuning a radio receiver of the type described, the character of the reproduced sound is employed as an indication of proper tuning. Where the response characteristic of the signal ampliiiers -is relatively flat topped, it ordinarily is not easy to determine when the local oscillator is exactly tuned to produce the predetermined intermediate vfrequency solely on the basis rof the amplitude of the reproduced sound. In many cases an approximation may be made on the basis of the quality of the reproduced sound. However,

in the case of a high iidelity radio receiver having signal amplifiers providing wide-band frequency response, the point at which the receiver is exactly tuned is quite diiiicult to determine by such means. If the receiver is not properly tuned, there is encountered what is known as a sideband cutting, and also the receiver is subject to signal interference from adjacent channels.

It has previously been proposed to control'some of the signal amplifiers, particularly the audio frequency amplifier, in such a manner `as to change a condition of operation thereof, as by reducing the gain therethrough, or otherwise to render them inoperative in certain cases, except in response to the tuning of the radio receiver to substantial resonance with desired or selected incoming modulated signals or carrier waves. In many cases, such muting control of the signal channel has been used not so much for the purpose of ensuring exact tuning of the receiver to the selected carrier waves, but more for the purpose of minimizing noise signals otherwise normally reproduced when the receiver is being tuned from one signal channel or station to another. Such systems have been provided particularly in conjunction with automatic volume control facilities, the effect of which is to render thereceiver extremely sensitive in the absence of strong received signals. Accordingly, when the receiver is tuned to a point between adjacent car-- rier channels, the sensitivity is increased so much that tube and other noise effects are unduly( amplified and reproduced in considerable volume.v Most systems which have been used for such pur-l poses operate in response to the amplitude of Accordingly, such' the received signal effects. systems are of no particular use for the purpose of determining exact tuning of the receiver, forl the reason that the flat, double-peaked and similar response characteristics of the amplifiers ren' der them incapable of making any substantialdiscrimination between different signal frequencies on the basis of their respective amplitudes.

It, therefore, is an object of the present invention to provide a tuning aid for radio receiving systems and the like by which to ensure in all' cases, irrespective of relative signal strengths,

tion of the system may be controlled in a desired manner, or rendered substantially unresponsive toreceived signals at all times except when the 'system is substantially eXactly tuned to an incoming signal.

A further object of the invention is to provide a tuning aid for a radio receiver which includes a signal frequency-responsive niuting system for a signal amplifier portion of the receiver, such as the audiox frequency amplifier portion, which is responsive only t the "signal frequency impressed thereon when the receiver is tuned for resonance with an incoming modulated carrier wave.

Still another object of the invention is to provide a muting system for an"andiofre'quencyv signal amplifier which norn'frally` ol'oerates to maintain the amplifier in anunresponsiveicone dition to any audio frequency` signals impressed thereon, but which, in response to the tuning of the radio receiver to a predetermined carrier Wave frequency, conditions the arrpli'e'zior opl erative response to audio signals. i

A still further object ofthe invention is to provideafrequency-responsive mutingfsystem for an audio-frequency signalampliiier-wherein the amplifier, when' conditioned forl operation, r func tions entirely independentlyof the frequency characteristicsof the muting-control circuits.

In accordance with the` present invention, there isprev'ideda tuning aid adapted for use with high iidelity=- radio -v receiving systems andv 'the like, in which a'sig-nal-amplifying portion of the system is-Y rendered inoperativev by gain-reducing or other` means and-vis' conditioned for normal operation by' frequency-responsive means when the system properly. tuned f to a selected signal.l In-'one embodiment thereisincluded any audio frequency` ampli-nery having input andA output circuits,` together with mean-s for impressing'upon the inputcircuit -audioI frequency signals which it isdesired toreproduce in amplified formin the-outputv circuit.- In addition,- there is pro'- videdmutingmeanscoupled to the input" circuit and-normally operative to render theam-piiier incapableof f responsetoan-y audio signals impressedupon the input circuit; Finally, asa contro-l system for-the mutingmeans, there is provided frequency-responsive means coupled-to the source 'of the signal-modulated carriervv waves and operative onlyin vresponse to ai predetermined carrier-wave-frequency forthe-purpose of ren--A dering: the rnutin-g4 means inoperative; .whereby tocondition the-*amplifier for response tothe audio frequency-signals impressed upon the input circuit thereof.

More-particularly; the invention mayl readily be embodied in 'a signal .receiving` system where-4 iny ari-audio'frequencysig-nal amplifier is pro-- vided with'biasing orother gain'contrel voltage which, when eifective, biases the-amplifier-'toor' beyond--cut-off; The sourceV ofbiasing. voltage may include a'control tube circuit which Iisre-V ducting forsignals in a relatively narrow bandf which are -thenutilized to effect the developmentv ofpredetermined control voltages for` the con'-v trol tube which operates to v`reduce the amplifier biasing voltage to a point at which the amplifier is rendered responsive to the audio frequencvsig- 'nals impressed'upontheinput circuit th'ereofE 'I he novelfeatures that are consideredfcharacter'istic of this invention'are- Vsetforthy with-4 particularty in the appended claims.` The inve'ifi'tion'V itself, however,A both as to its'org-aniza# tion and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description taken in connection with the accompanying drawing, in which:

Figure l is a circuit diagram of that portion of asradioreceiver embodyinge the-present invention, and

Figure 2 is a response curve of the intermediate frequency amplifier which may be used in a radio receiver embodying the instant invention.

Referring'nowto Figure l of the drawing, there is shown'asoure of signal-modulated carrier waves which,in-the-present instance, is represented'v by: an'`vintermediate frequency amplier l0 which, in-its--ou'tpiit circuit, includes a resonant network Il comprising the parallel arrangement of "al coill'and aresonating capacitor i3, whereby-thenetwork is tuned for response to a predetermined intermediate frequency. A second rescnantnetworkA I4, cornprisingva coil 51- inductivelycoupled tothe coil'i?" and shunted by a resonatingv, capacitor |76, also is tunedfr resonance' at the predetermined intermediate frequency. The coils-l2 and I'form the-'primary and secondary windings, respectively, of an-intermediate frequency transformer l 1.

.Therey also lis provided an electron discharge devicey I 8 which, for convenience, mayjbe a duplex diode-triode which preferablyv is lla high -mu -de- Vice-such asl-the RCA type-SMITS.' It isv-to-be-un derstood that the invention is-not limitedtothe use of a tube having the triodeand ther4 two diodes -in the same envelope. Any high mu Atriode may be Vused with a separate duplex-diode or with two separate diodes,Y for example.- It'is vintend-'- ed,- therefore, that subsequent references to electronicl devices Yof'these characters Yas triodes anddiodes-jbe construed to include anyY of the aforementioned alternative forms.- The high' potential-terminal of the seconda-ry resonant net- Work Il! Ais-cmJpled to a'diode anode I9-which, to gether with acommon cathode-2, :comprises the signal-demodulating tubeV or signal detector of a-radio receivera The `'cathode 20 is connected-fte groundthrough--abiasing resistorl 2| I which, sofar asthesignal detector is concerned, vis of'no particular importance. The-load circuit'for the signal detectoryincludes the series arrangementlof resistors 22-and -23- coupled between Ithe low potential terminal of the secondary resonantV net-- Work Alll Vandsground. T-histerminal also is maintained -at-vground potentialfor carrier wave fre# quencies by means of a=capaci-tor 24; The junc" tion pointbetween the signal 4detector load resistors22and 21B-'is bypassed? to 'ground forI carrier: wave frequencies by/a capacitor 2 Bland is fcoupled by a-capacitor 26 to rvolume control device 211; Thesliding contact'vZ-Sfof the volume controlfdee vice is coupled by,k means ofa capacitory 29\ tov the control grid l39-A of the triode i section vvof the` tube Illi which functions as aiI signal-amplifying; tube, or more particularly as fthe-first stagefoi an audio frequency: signalamplifierl A leak' resistor 3i isfcoupledbetweenthe control-grid 30-and grou-nd The'anode-Sof the tube l 8may.: begconventionallyfcoupled; suchfas-by a capacitor' 33-,to-further audio-frequency amplifier stages." Spacecurrent for th'etriode section of thef tubev` |8\ is' suppliedf from= a" source of' unidirectional energy'such asa `battery 34"; the negative terminal of which: is grounded and the: positive terminal-I offwhichfis coupledtothe anode 3`2 through av loadresistor 3 5;

The tube i8 alsois provided Withanother diodel anode 36 which is coupled to the high potential terminal of the secondary resonant network I 4 by a resonant device or resonator such as a piezoelectric crystal 31. The frequency characteristics of the crystal should be so chosen that it will respond only to the impression thereon of a predetermined frequency which, in the present instance, should be the center frequency of the intermediate frequency pass band. The diode formed by the anode 36 and the common cathode 20 constitutes a carrier wave-rectifying tube and is provided with a load circuit consisting of a resistor 38. The anode 36 also is coupled, through a filter network includingl a series resistor 39 and a shunt capacitor 40, to the control grid 4I 0f a control electron discharge device such as the triode tube 42 functioning as a biasing voltage-developing tube. This tube preferably is a low mu device such as the RCA type 604. The filter including the resistor 39 and the capacitor 4D is for the purpose of removing the audio frequency signal components from the voltage developed in the rectifying diode load resistor 3B, so that there is impressed upon the control grid 4I a unidirectional voltage, the amplitude of which is independent of the instantaneous values of the audio signals. The cathode 43 of the tube 42 is coupled to the common cathode 20 of the tube I8, and thence through the biasing resistor 2| t0 ground. The common cathode biasing resistor 2| is bypassed for alternating currents by a capacitor 44, The anode 45 of the tube 42 is coupled directly to the positive terminal of the battery 34 by which the tube is supplied with space current.

The following description of the operation of the system embodying the present invention will betterbe understood by having additional reference ot Figure 2 of the drawing. There is shown a curve 46 representing the response of the intermediate frequency amplifier over the band of frequencies which it is designed to pass. As will be seen, the curve 46 is of the double-peaked type and has a relatively flat response over a substantial band of frequencies which, for the present illustrative purpose, will be assumed to be of the order of 15 kilocycles. The response curve 46 also will be observed to be substantially symmetrical va-bout the center intermediate frequency 4l.

Assume that, in the first instance, the local oscillator (not shown) is adjusted so that, when the frequency output thereof is combined with the received carrier waves, the intermediate frequency so produced is at one side or the other of the center intermediate frequency represented by the vertical line 4'I of Figure 2. For example, assume that the intermediate frequency produced by the mistuning of the local oscillato-r is represented by the vertical line 48. It will be seen that, at this frequency, response by the intermediate frequency amplifier is substantially at a. maximum. The signal-modulated intermediate frequency carrier waves impressed upon the signal detector including the anode I9, therefore, have a maximum amplitude. However, by reason of the operation of the system embodying the present invention, no response by the sound reproducing facilities is produced by these signal eifects This may be seen by considering that, at this time, the biasingvoltage developing tube 42 is so controlled that it is conducting space current of considerable magnitude. This current, in traversing the biasing resistor 2 I, develops at the cathode 20 of the tube I3 'a voltagevof positive polarity relative to ground and of sufficient magnitude to bias the triode signal-amplifying section ofthe tube I8 beyond the point of space current cut-olf. At the same time, in the illustrative embodiment of the invention where the cathode 20 is associated not only with the triode section ofthe tube I8 but also with the diode anode I9, this positive voltage also effectively biases the diode I9-20 so that it will not respond to any signal effects impressed thereon. It will be appreciated that these conditions obtain by reasonof the fact that, at the assumed frequency represented by the line 48 of Figure 2, the crystal 3l is completely unresponsive. It is responsive only to those frequencies lying within the relatively narrow band represented by the shaded area 49 at the center of which is the center intermediate frequency 4l. Under vthese conditions the rectifying diode 36-2 is'non-conducting and there is no voltage developed in the load resistor 38.

As the adjustment of the local oscillator is changed to produce an intermediate frequency which lies within the range represented by the shaded area 49 of Figure 2, which may conveniently be made equal substantially to 1/2 kilocycle, the crystal 3l responds and becomes a relatively low impedance device coupling the anode 36 to the network i4, whereby to impress the signal-modulated intermediate frequency carrier wave energy-developed at the high potential terminal of the secondary resonant network I4 upon theanode 36. The carrier Wave-rectifyingdiode 35-23 thereby ybecomes conducting to develop a uni-directional voltage* in its' load resistor 38. This voltage, after the Vlter 39-40 functions to remove ltherefrom the audio frequency components, is impressed in negative polarity upon the control grid 4i of the biasing voltage-developing tube 42. In response to the impression of the rst increment of this voltage upon the control grid 4I, space current conduction in the tube 42 is decreased, thereby effecting a corresponding incremental decrease in the magnitude of the positive voltage developed in the biasing resistor 2|. Thus, Whatever biasing effect that the irnpression of this positive voltage upon the cathode 20 of the tube I8 has upon the operation of the rectify'ing diode :i6- 2o is decreased, thereby further'increasing the conductivity of this diode. The operation of the system, accordingly, is seen to be cumulative sothat there is produced what is effectively a trigger action, the result of which is to reduce substantially instantaneously the biasing voltage Vat the cathode 20 to an ineffective magnitude. The diode I9-20 thereby becomes responsive to the signal-modulated carrier waves impressed thereon from the resonant network I4 to develop demodulated signal effects in the load resistors 22 and 23. Also, at the same time the triode section of the tube I8 is rendered responsive to amplify the demodulated signal effects impressed upon the control grid 30 from the load resistors 22 and 23. In this manner the radio receeiver may be quite accurately tuned for response to a selected radio frequency carrier wave.

It will be lappreciatetd that the crystal 3I may be selected so asfto restrict its response to such a narrow band of intermediate frequencies that it will be leffective to disable or mute the audio frequency signal amplifier vat all times except when the receiver is tuned to produce an intermediate frequency within the lresponse band of the crystal. It also Will be appreciated that, once having adjusted the frequency of the local oscillator to produce an' intermediate frequency lying within manner.

the response range of the crystal 31, the amplifying system operates :in an entirely vconventional For. example, the 'pass band of frequencies, which is demodulated by the signal detector diode lil- 20, is determined principally by the characteristics offcircuits other than that including the crystal 31, such as. for example, the primary and secondary :resonant networks Il and and l Il of the intermediate frequency transformer l1.

The behavior of the receiver is comparablek to one which is extremelyselective, but yet, once it is -properly tuned, it Ahas Whatever characteristics may-be desired to reproduce an audio Vsignal with optimum fidelity. By the use of-such a device as that provided by the instant invention, sideband cutting, whereby signal distor-tionafiecting quality of the reproduced sounds is 4produced, may be entirely eliminated. Also, any signal interference from adjacent channels, which is encountered frequently -iu a radio receiver as a result of inistuning, may be completely elim inated.

It will be obvious to thoseskilled -in the art that the vpresent invention is susceptibleA of embodiment in forms other than that specifically shown yherein `for purposes of illustration. As already indicated, the tube I Smay bereplaced by separate tubes, one of which being a highmu triode and the other of which being either a duplex diode or two separate diodes. In the event that it is desired to use separate tubes, they may be interconnected substantially as 'shown herein; that is, with the cathodesall connected together so that, as far as the external circuits are concerned, they will function substantially as the common cathode 20. However, it also will be appreciated that it is not essential that the cathode cooperating with the signal demodulating rectier suchas ISL-20 and the cathode associated Vwith the triode signal amplifier both be biased by means of the voltage developedin the biasing resistor "2L lIt vmay be desirable only to bias the signal-demodulating rectifier so as to render it unresponsive to signal-modulated carrier waves impressed thereon v'fromthe resonant circuit T4 until the receiver is "accurately tuned to render the rectifying. diode operative, whereby to'remove the bias from the'demodulatingdiode. Alternatively, the cathode associated with lthe signal amplifying Ytriode Yonly inayk be biased so as to render'this tube unresponsive to any signals which may vbe impressed thereon from the demodulating diode load circuit resistors 22 and 23. In either case, 'the audio .signal-amplifying system is completely unresponsive until the receiver is accurately tuned. Furthermore, the system vwill operate in an entirely satisfactory manner if the cathode associated withthe .rectiiying diode is not subject to the bi'asing'voltage developed by the tube 42. In such a case thetr'i'gger action resulting from the circuit arrangement shownand describedjherein'will not be as 'pronounced. However, for all practical vpurposes the audio signal-amplifying system will be rendered operative substantially instantaneously'as soon as the receiver isaccurately tuned substantally to the 'center 'intermediateirequency Also, it "will'be evident that the'invention'is not limitedto "embodiments thereof includingA a piezoelectric crystal v.as theirequency-responsive means 4by 'which themuti'ng -circuit oilthe audio signal `amplifier is rendered inoperative,`whereby to condition the amplifying .system v for response to the audiosignals. Otherreso'nant bodies, such as magnetostrictve devices, may be used with substantially equal facility without departing from the spirit of the invention. Also, it isconsidered to be within the scope `of the invention to utilize for such -a purpose a frequency-responsive network such as a narrow band pass viilter, for example. The only essential requirements are that the means employed for such a purpose be rather critically responsive toa predetermined relatively narrow band Aof frequencies and that, in response to such frequencies, it befcapable of coupling the rectifying diode to the source of the signal-modulated carrier waves.

What is claimedA is:

l, A tuning aid adapted for use with high delity radio receiving systems and the like, `comprising a singe tuned signal input circuit responsive tg signal-modulated carrier waves covering a vrelatively wide band of freqencies,ari-electronic signal-demodulating tubecoupledto said signal input circuit and having an output Vcircuit in which to develop said signals, an electronic signal-'amplifying tubehaving an input circuit and a cathode circuit coupled to saidoutput circuit, an Ielectronic carrier wave-rectifying tube having an output circuit, an electronic-biasing voltagehdeveloping tube having an input circuit coupled to the output circuit of said rectifying tube and an output circuit Iin which to develop vsaid biasing voltage coupled .to ther-input `circuit-of saidy amplifying tube, Vandra lcircuit including a piezoelectric crystal resonant andeiiective only at 'a predetermined carrier lwave frequency connected between one 'terminal of said signal input circuit and said rectiiying tubeto'couplezsaid riectifying tube to said signal `input circuit.

2. tuning aid'adaptediforuse inhighiidelity radio receiving systems and 'the like, comprising a single "tuned `signal input circuit responsive to .signal-modulated carrier waves,l 'signal-demoniulating means including valrstdiode and .a first load virripedance vdevice therefor coupled to lsaid signal `input circuit, .signal-amplifying means including z-1triode having a `cathede andaspace currentcontroliing electrode coupled tosa'id first loadinipedance device, a biasinimpedance device l connected lto said "cathode, carrier waverectifying means including la second diode' and a second load impedance device therefor, `biasing voltage-'developing means including a vsecond triodehaving .a space current-controlling electrode coupledto said second load impedance device and a cathode `connectedto thecathodeof said first mentioned triode. and ineens including a piezoelectric crystal resonantat-a predetermined vcarrier wave .frequency and connected vbetween :said first diode and said carrier wave-rectifying means to `effectively couple said signal input circuit only at said ,predetermined frequency.

3. A tuning aid adaptedior use in high fidelity radio .receiving` systems and the like, comprising a single ltuned signal input circuit responsive to signale-modulated wide band intermediate frefluency carrier waves, a signal-detecting diode and a load circuit resistortherefor-coupled to said signal-input circuit, .a signal-amplifying triode having a cathode and a control grid coupled to said signal-detecting diode load resistor, a zbiasing resistor for saidtriodevconnected to said cath ode, arcarrier wave-rectifying diodeand a load resistor therefor, a ybiasing *voltage control triode havinga grid coupled'to saidcarrier wave-,rectifying-diode load resistor and Vva cathode connected to said .first mentioned vtriode circuit, vand a Vpiezoelectric crystal resonant at-said intermediate frequency connected between said signal input circuit and said last named diode for coupling said carrier wave-rectifying device to said signal input circuit only at said intermediate frequency.

4. A control system for signal amplifiers comprising a source of signal modulatedcarrier Waves in a predetermined band of frequencies, demodulating means coupled to said carrier Wave source, a modulation signal amplifier having an anode and cathode and an input circuit connected with said demodulating means to receive the signal output therefrom, voltage supply means for supplying a biasing voltage connected with said amplifier input circuit, said voltage supply means comprising an electron tube having an anode and cathode connected respectively to the anode and cathode of said amplifier, a rectifier for controlling the biasing voltage applied to said amplifier from said supply means, said rectifier connected between said biasing voltage supply means and said source of carrier Waves, said biasing voltage from said supply means being connected With said rectifier to render said amplifier and said rectifier inoperative to conduct signals, means responsive to a predetermined center frequency within said band of frequencies connected between said rectifier and said source of carrierwaves to control said biasing voltage in a direction to render said amplifier operative and responsive to signals at said predetermined center carrier-wave frequency.

5. A control system for signal amplifiers comprising a source of signal modulated carrier Waves in a predetermined band of frequencies, means coupled to said carrier Wave source for demodulating said carrier waves, a modulation signal amplifier having an anode and cathode and an input circuit connected with said demodulating means to amplify signals therefrom, voltage supply means for supplying a biasing voltage connected with said amplifier input circuit, said voltage supply means comprising an electron tube having an anode and cathode connected respectively to the anode and cathode of said amplifier, and a rectifier for controlling said biasing voltage connected between said biasing voltage supply means and said source of carrier waves, said biasing voltage from said supply means being connected with said rectifier and said demodulating means to render said amplifier and said rectier and said demodulating means inoperative to conduct signals, means responsive to the center frequency Within said band of frequencies connected between said rectifier and said source of carrier Waves to control said bias voltage source, whereby said amplifier is rendered responsive to signals at said carrier-Wave center frequency.

BENJAMIN S. VILKOMERSON.

References Cited in the flle of this patent UNITED STATES PATENTS Number Name Date 2,096,874 Beers Oct. 26, 1937 2,115,871 Pitsch May 3, 1938 2,250,596 Mountjoy July 29, 1941 FOREIGN PATENTS Number Country Date 422,858 Great Britain Jan. 21, 1935 425,412 Great Britain Mar. 11, 1935 443,956 Great Britain Mar. 10, 1936 461,966 Great Britain Feb. 22, 1937 

